Who knows the mind of an ECM?

What is it thinking? It, this ECM, is opening the digital EGR valve (three solenoids) at idle. That’s a no-no, of course. The park / neutral switch only makes contact in park and neutral. The power steering switch is NOT making contact (not providing a ground when it shouldn’t). The patient is a 1990 Pontiac Grand Prix LE 3.1L, non-turbo. It will only idle if the fuse for the EGR valve/ purge control valve is out. It does have fuel pressure, and spark, and sensor inputs (correct values) during idle. No, I don’t have a Tech 1 scan tool.

Are you sure there isn’t a short in the EGR circuit, causing the EGR to open whenever the ignition is on? Are you positive the ECM is sending the signal to open the EGR when the engine is idle? Most ECMs operate easy-to-replace relays that actually trigger the operations of components, like the fuel pump, EGR, cooling fans, etc. These relays can got bad, and stick on or never go on. Check for the relays before you replace the ECM.

It sounds to me that there is a circuit problem in the ECU. Have you tried replacing it.

I did suspect a short. I checked the resistance from the ECM connectors (disconnected from the ECM) through the EGR’s three solenoids and got the values one should get of 16 to 28 ohms each. When connected to the ECM, the voltages were near ground potential (which it would be when the ECM is causing the voltage to drop by closing its contacts to ground). There are no relays in this circuit. Power comes from an all-the-time-hot fuse, goes through the EGR solenoids, and goes to the ECM. The ECM grounds the circuit(s) at the (hopefully) appropriate time, and this energizes the solenoids. According to the scanner chart, if the park / neutral switch isn’t closed when in park or neutral, or if the power steering switch IS closed, the EGR valve will be energized. Neither happened. I wonder if there is ANOTHER condition which will cause the ECM to energize the EGR valve at idle. …

I would use another ECM as a substitute; but, I don’t have one. I would use a Tech 1 scan tool; but, I don’t have one. All of the shops, over the years, which did have the scan tools didn’t fix the problem; even though they charged for “computerized diagnotic”. Since I’ve narrowed the problem, maybe, a good shop can be found which will be able to fix it.

A 1990 computer may still be cheap. I don’t think it is, but the 85’s were. The one I tried was a Sorensen reconditioned that I got at Kragen for around a hundred dollars. My computer problem was very obvious and I have no idea if yours is bad or not. It’s too bad the EGR system isn’t as simple as it used to be. I could have been a good mechanic in the seventies.

You can purchase a remanufactured ECM from partsamerica.com (Schucks) for $90.
I can buy one locally used for $50
It sounds like you need a replacement.

Thanks, guys. I think I will tell my neighbor to try another ECM. He’ll probably agree, even though it’s a chancy decision. [Don’t touch that PREVIEW REPLY button. It’ll vaporize your reply. It has vaporized several of my replies.]